Insertion compounds can be defined as those compounds wherein an amount of an element, molecule, or other species can be inserted into the host structure of the compound and then removed again without having irreversibly altered the host structure. Thus, while the host structure may be altered by insertion of a species, the original structure is retained upon subsequent removal of the species. Generally, only minor alterations of the host structure can occur before insertion is no longer reversible, although there are many examples of reversible phase transformations in the literature.
Insertion compounds have proven useful for a variety of applications such as use as ion exchangers, but they are particularly suitable for use in non-aqueous rechargeable batteries. The excellent reversibility of some of these compounds upon insertion with lithium makes such compounds very attractive for use as electrodes in lithium rechargeable batteries. Two manufacturers, Sony Energy Tec. and AT Battery, have made lithium-ion type batteries commercially available wherein both the cathode and anode electrodes are lithium insertion compounds. In each case, the cathode is a lithium cobalt oxide compound and the anode is a carbonaceous material.
Typically, lithium-ion type batteries are constructed using components that may be somewhat sensitive to water vapour but are otherwise stable in air. Thus, the batteries can be assembled economically under dry air conditions at the worst. It is therefore important to choose electrode materials that are air stable. Lithiated carbonaceous material anodes are not stable in air, so batteries are usually made in a completely discharged state wherein all the lithium in the battery resides in the cathode. Preferable cathode materials therefore have the maximum possible amount of lithium inserted while still being air stable. Additionally, cathode materials preferably are chosen that allow the maximum possible amount of lithium to be reversibly removed and re-inserted, hence providing the maximum battery capacity.
Many lithium transition metal oxide compounds may be used as cathodes in lithium-ion battery products. Along with LiCoO.sub.2 (used in the Sony Energy Tec. product and described in U.S. Pat. No. 4,302,518 of Goodenough), other possible compounds include LiNiO.sub.2, (also described in the aforementioned U.S. Patent), LiMn.sub.2 O.sub.4 (described in U.S. Pat. No. 4,507,371), and other lithium manganese oxide compounds. Since cobalt is relatively rare, LiCoO.sub.2 is relatively expensive compared to the latter two compounds. Both Co and Ni containing compounds are considered to be potential cancer causing agents and are therefore subject to strict handling requirements, particularly with respect to airborne particulate levels. Lithium manganese oxides are less of a toxicity concern and are relatively inexpensive. For these reasons, such oxides would be preferred in commercial lithium-ion type batteries if other performance requirements can be maintained.
Another attractive Li--Mn--O compound for use in lithium-ion batteries is Li.sub.x MnO.sub.2 having a .gamma.-MnO.sub.2 type structure wherein x can range approximately between 0 and 1. The Li.sub.x MnO.sub.2 compound can be synthesized from suitable precursor materials (see U.S. Pat. No. 4,959,282) but only for values of x between approximately 0.33 and 0.43. Further lithium can be inserted and reversibly removed electrochemically as described in the aforementioned '282 patent. Other Li--Mn--O compounds considered in lithium-ion batteries include Li.sub.2 Mn.sub.2 O.sub.4 and Li.sub.4 Mn.sub.5 O.sub.12 as described in M. M. Thackeray et al, J. Electrochem. Soc., 139, 363 (1992).
A Li--Mn--O compound denoted Li.sub.2 MnO.sub.2 and having a layered structure described by the space group P-3m1 is known to exist. The lattice constants for this compound are a=3.195 .ANG. and c=5.303 .ANG. (W. I. F. David et al, Revue de Chimie Minerale, 20, 636 (1983)). However, it is not known from the literature whether lithium can be removed from Li.sub.2 MnO.sub.2, electrochemically or otherwise, nor what would happen to the host structure if such removal were possible.
To enhance the operating capacity of lithium-ion type batteries, it has been considered desirable, where possible, to insert additional lithium into the cathode material using chemical means prior to battery construction. For example, LiMn.sub.2 O.sub.4 with the spinel structure can be further lithiated reversibly up to a stoichiometry of Li.sub.2 Mn.sub.2 O.sub.4 using a reaction involving LiI as described in U.S. Pat. No. 5,196,279. However, iodine compounds can be quite corrosive and this creates potential problems when contemplating such a process for large scale manufacturing. Li.sub.x MnO.sub.2 with the .gamma.-MnO.sub.2 structure might be further lithiated to Li.sub.1 MnO.sub.2 using a similar process. Use of this latter compound would provide very high capacities in lithium-ion batteries of conventional construction.
An alternative method to further lithiate conventional insertion compounds would be to electrochemically insert the lithium. This could be accomplished using an electrochemical cell to process (or lithiate) a starting insertion compound. With lithium metal as an anode, the starting insertion compound as a cathode, and a suitable non-aqueous electrolyte comprising a lithium salt, a controlled discharge of the cell would result in the desired further lithiation of the starting insertion compound. However, such a process is prohibitive on a manufacturing scale, in part due to the use of highly reactive lithium metal.
Lithium transition metal oxides are generally not stable in air. Only if the lithium atoms are sufficiently tightly bound to the host will they not react with water vapour, oxygen, or CO.sub.2 in the air. A direct measure of the binding energy of the lithium atoms in a lithium transition metal oxide is the voltage of said oxide with respect to lithium metal in a non-aqueous battery. Empirically, it has been determined in J. R. Dahn et al, J. Electrochem Soc., 138, 2207 (1991) that lithium insertion compounds are effectively air stable if the voltages of said compounds versus lithium are greater than 3.3.+-.0.2 V. Li.sub.2 Mn.sub.2 O.sub.4, with a voltage versus lithium near 2.8 V, reacts even with the moisture in the air to form LiOH and LiMn.sub.2 O.sub.4. Similarly, Li.sub.1 MnO.sub.2 having the .gamma.-MnO.sub.2 structure reacts with moisture in the air. While it is possible to construct a lithium-ion battery with cathode materials like these, special handling and storage procedures are required to minimize the reaction with air to an acceptable level in practice. Generally, it would be expected that direct exposure of these compounds to an aqueous environment would result in serious degradation of the compounds via reaction of the lithium with water.